Method for monitoring a work vehicle suspension

ABSTRACT

Off-highway trucks generally rely on a suspension system which employs a plurality of gas-over-liquid type struts. These struts are critical to proper operation of the vehicle such that a single collapsed strut can have serious manifestations in structural damage, tire wear, and payload monitor accuracy. These consequences can be mitigated by an accurate and reliable strut monitor. Pressure type sensors are disposed on each of the struts and their pressure is monitored during three critical phases of operation. These phases include static, loading, and roading modes and each mode requires a distinct method for detecting a collapsing strut. The presence of a collapsing strut, detected by any of the three methods, is communicated to the vehicle operator whereby operation can be immediately suspended. The system avoids the serious consequences of vehicle operation with a collapsed strut by providing the operator with immediate and positive feedback on the condition of the struts.

DESCRIPTION

1. Technical Field

This invention relates generally to a method for accurately determiningthe condition of a work vehicle suspension, and more particularly to amethod which detects a collapsed strut by monitoring strut pressures.

2. Background Art

In the field of off-highway trucks used, for example, in miningoperations, it is desirable that an accurate record be kept of thequantity of material removed from the mining site. This information canbe used to calculate mine and truck productivity as well as aid inforecasting profitability and work schedules.

Prior systems, as disclosed in U.S. application Ser. No. 749,607 filedJune 25, 1985 by D. Foley et al., have shown that strut pressure can bean accurate indicator of payload. The apparatus disclosed thereinincludes an electronic control which monitors each of the strutpressures, compensates for various inaccuracies introduced by loaddistribution and vehicle attitude, and correlates this information intoactual payload. However, proper operation of the payload monitorrequires that all of the struts be in good working order. For example,theoretical calculations for a particular family of struts have shownthat a loss of 125 ml of oil from a single strut can generate a 22%error in calculated payload. No provision has been made for monitoringthe condition of the struts and indicating the condition of a faultystrut.

Prior systems have relied upon the vehicle operator to visually inspecteach of the struts before operating the vehicle. This practiceintroduces considerable subjectivity into the system and results in thevehicle being operated with partially or completely collapsed struts.Both operator inattention and inability to recognize a collapsed strutare contributing factors to erroneous operation; however, even withproper inspections the strut may also collapse during operation. Inlarge off-highway trucks, a single collapsed strut will not havesignificant effect on the "feel" of the truck and can easily gounnoticed by an experienced operator.

Operation of the vehicle with a collapsing strut will have obviouseffects on the accuracy of the payload monitor owing to the change inthe relationship between strut pressure and payload. Other seriousconsequences also result from such operation. For example, uneven tirewear is an undesirable result of extended vehicle operation with acollapsed strut. Tires are a major operating expense of off-highwaytrucks and any change in their replacement schedule can have seriousimpact on profitability. Thus, a collapsed strut can have economicimpact other than replacement of the damaged strut. Moreover, acompletely collapsed strut results in repeated metal-to-metal contactand the possibility of eventual major structural failure. Frame damagecan occur after relatively short periods of operation and the resultantrepair costs can be exorbitant.

The present invention is directed to overcoming one or more of theproblems as set forth above.

DISCLOSURE OF THE INVENTION

In accordance with one aspect of the present invention, a method fordetecting a collapsed strut of a work vehicle which has a plurality ofleft and right strut mounted wheels includes the steps of periodicallysensing the internal pressure of selected struts and delivering aplurality of first signals each having a magnitude correlative to theinternal pressure of each respective selected strut. The method furtherincludes the steps of deriving an indication of the condition of each ofthe struts responsive to the pressure of the selected struts anddelivering a signal indicative of a collapsed strut in response to thepressure signals being outside a preselected range.

In accordance with another aspect of the present invention, a method fordetecting one of a collapsed strut and an underinflated tire of a workvehicle which has a plurality of left and right strut mounted wheelsincludes the steps of periodically sensing the internal pressure of eachof the struts and delivering a plurality of first signals each having amagnitude correlative to the internal pressure of each respective strut.The method further includes the steps of comparing consecutive firstsignals of each respective strut and delivering a second signal having amagnitude correlative to the differential therebetween, counting thenumber of second signals exceeding a preselected setpoint during apreselected period of time, comparing the count for each strut to thecount for another strut and delivering a third signal in response to thecount differential exceeding a preselected setpoint. The counts of thesame side adjacent struts on the side of the vehicle associated with thelowest count are compared in response to receiving the second signal anddelivering a signal indicative of one of a collapsed strut and anunderinflated tire in response to the count differential exceeding apreselected setpoint.

In accordance with another aspect of the present invention, a method fordetecting a collapsed strut of a work vehicle which has a plurality ofleft and right strut mounted wheels includes the steps of periodicallysensing the internal pressure of selected struts and delivering aplurality of first signals each having a magnitude correlative to theinternal pressure of each respective selected strut. The method furtherincludes the steps of calculating individual strut displacement inresponse to the magnitude of the first signals, comparing the calculatedstrut displacement to a desired displacement and delivering a signalresponsive to the difference therebetween. A signal indicative of acollapsed strut is delivered in response to the differential exceeding apreselected setpoint.

In accordance with another aspect of the present invention, a method fordetecting a collapsed strut of a work vehicle which has a plurality ofleft and right strut mounted wheels includes the steps of periodicallysensing the internal pressure of selected struts and delivering aplurality of first signals each having a magnitude correlative to theinternal pressure of each respective strut. The method further includesthe steps of storing a first set of the periodically delivered firstsignals in response to the first signals remaining substantially stableat a first magnitude for a preselected duration of time, storing asecond set of the periodically delivered first signals in response tothe first signals remaining substantially stable at a second magnitudefor a preselected duration of time, computing the stiffness for each ofthe struts in response to the difference in magnitude between the firstand second sets of periodically delivered first signals, comparing thestiffness of each of the struts to the stiffness of another of theselected struts, and delivering second signals each having a magnitudecorrelative to the stiffness differential. A signal indicative of acollapsed strut is delivered in response to the differential exceeding apreselected setpoint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagrammatic view of an off-highway truck and thelocation of critical suspension components;

FIG. 2 illustrates a block diagram of the suspension monitor;

FIG. 3 illustrates a portion of one embodiment of the software flowchartfor implementing the suspension monitor during a static portion of atruck cycle;

FIG. 4 illustrates a portion of one embodiment of the software flowchartfor implementing the suspension monitor during a loading portion of atruck cycle; and

FIGS. 5A and 5B illustrates a portion of one embodiment of the softwareflowchart for implementing the suspension monitor during a roadingportion of a truck cycle.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, wherein a preferred embodiment of thepresent apparatus 10 is shown, FIG. 1 illustrates a work vehicle 12which can be, for example, an off-highway truck 14. The truck has atleast one front and rear strut 16,18 disposed in supporting relation toa load carrying portion 20 of the work vehicle 12. The preferredembodiment has two front and two rear struts 16L,16R;18L,18R which arethe gas-over-liquid type commonly known in the industry and notdescribed herein. It is sufficient in the understanding of the instantapparatus 10 to recognize that the pressure of the fluid is indicativeof the magnitude of the load applied to the strut 16,18 and that wideswings in the strut pressures are normal and even expected during actual"roading". Moreover, a strut which has lost pressure and collapsed showslittle response to "roading" with significantly less variation in strutpressure. Conversely, an underinflated tire will increase the frequencyof the strut pressure variations within the strut supporting that tire.The underinflated tire has a lower spring coefficient than a properlyinflated tire and will resultantly increase the oscillatory response ofthe suspension with corresponding variations in the damping strutpressure.

The load carrying portion 20 includes a vehicular frame 22 and dump body24. The dump body 24 is connected to the frame 22 by pivot pin 26 and ahydraulic cylinder 28 such that the contents of the dump body 24 can beremoved by controllably pressurizing the cylinder 28 to effect pivotalmovement of the dump body 24 about the pivot pin 26. In the transportmode, the cylinder 28 is not pressurized and the weight of the dump bodyis transferred to the frame through the pivot pin 26 and a support pad30 fixed to the frame 22.

The work vehicle 12 further includes a ground engaging portion 32 and asuspension means 34 for supporting the load carrying portion 20 in amanner to provide damped oscillatory motion between the ground engagingportion 32 and the load carrying portion 20. The suspension means 34includes a rear axle housing 36 and an A-frame moment arm 38. TheA-frame moment arm 38 has a first end portion 40 pivotally connected tothe vehicular frame 22 by a socket 42 and a second end portion 44fixedly connected to the rear axle housing 36. The first end portion 40of the A-frame moment arm 38 is a king bolt arrangement, substantiallyspherical in shape and retained from lateral movement by the socket 42.The rear strut 18 has a first end portion 46 pivotally connected to thevehicular frame 22 and a second end portion 48 pivotally connected tothe second end portion 44 of the A-frame moment arm 38.

During load of the truck, as the payload increases, the load carryingportion 20 will be displaced in a direction toward the ground engagingportion 32. The rear strut 18 begins to compress while the A-framemoment arm 38 pivots about first end portion 40. A distance L2 isdefined to be the distance between the.first end portion 40 pivot pointand the second end portion 44 pivot point of the arm 38. Therefore, itcan be shown that the rear strut pressure differential is a function ofthe suspension means 34. Moreover, the rear strut pressure differentialcan be related to the reaction force R between a work surface and theground engaging portion 32. A force S experienced by the rear strut 18can be determined by measuring the internal pressure of the strut 18,subtracting the rear strut pressure corresponding to an unloaded truck,and multiplying the differential pressure by the area of the strut 18. Areaction force R is proportional to the payload of the vehicle 12 andcan be assumed to act through the center of the rear axle housing 36such that a summation of the moments about the pivot point of the kingbolt would derive the following equation:

    R=S * L2/L3                                                (eqn. 1.1)

where the horizontal distance between the first end portion 40 pivotpoint and the center of rear axle housing 36 is defined to be L3.

Similarly, the front strut 16 will be compressed as the load increases;however, the front strut is connected directly between the frame 22 anda front axle housing 50. A more straightforward relationship exists herein that a force F experienced by the front strut 16 can be determined bymeasuring the internal pressure of the strut 16, subtracting the frontstrut pressure corresponding to an unloaded truck, and multiplying thepressure by the area of the strut 16. The reaction force F between theground engaging portion 32 and the work surface is substantiallyequivalent to the force F experienced by the front strut 16.

The apparatus 10 is shown in FIG. 1 to illustrate the relationshipbetween the work vehicle 12 and the location of the apparatus 10. A moredetailed block diagram of theapparatus 10 is shown in FIG. 2 anddiagrammatically illustrates a means 52 which periodically senses thepressures of each of the struts 16,18 and delivers a plurality ofsignals respectively correlative to the magnitude of the internal strutpressures. The means 52 includes a plurality of pressure sensors54,56,58,60 of the type commercially available from Dynisco as partnumber PT306. The pressure sensors 54,56,58,60 are respectivelyassociated with the two front struts 16L,16R and the two rear struts18L,8R. Each of the pressure sensors 54,56,58,60 delivers an analogsignal proportional to the magnitude of the pressure of the respectivestrut 16L,16R,18L,18R to respective analog to digital converters (A/D)62,64,66,68. The A/D's 62,64,66,68 can be of the type commerciallyavailable from Analog Devices as part number AD575A. Other types of A/Dconverters have been contemplated by the inventor and the choice of theparticular A/D disclosed herein is simply a matter of designerdiscretion. The selection of a device which provides an analog tofrequency output is particularly well suited to the digitalmicroprocessor environment disclosed herein; however, other similardevices could be easily substituted without departing from the spirit ofthe invention.

A Motorola programmable interface array (PIA) 70 receives the digitalfrequencies output by the A/D converters 62,64,66,68 and delivers thesesignals to a microprocessor 72 under software control. In the preferredembodiment, the microprocessor 72 is part number 6809 provided by theMotorola Corp. The apparatus 10 also includes a means 74 which receivesthe control signal and delivers an indication of the magnitude of thework vehicle payload in response to the magnitude of the control signal.The indicating means 74 includes a second PIA 76 connected through adriver circuit 78 to a pair of individually energizable incandescentlamps 80,82. These lamps 80,82 are used to give indication to both thetruck operator and the operator of the loading equipment of the statusof the load relative to rated truck capacity.

A third incandescent lamp 84 is connected to the PIA 76 via the drivercircuit 78. The third lamp 84 is addressable by the microprocessor 72 toindicate either a collapsed strut or an underinflated tire and can beviewed primarily by the truck operator.

Referring now to FIGS. 3, 4, and 5, subroutines are diagrammaticallyillustrated via flowcharts which disclose software routines fordetecting a collapsed strut of a work vehicle having a plurality of leftand right strut mounted wheels. Each of the routines is associated witha particular phase of vehicle operation and will only be executed whenthe vehicle is detected to be operating in a preselected manner. Forexample, off-highway trucks are known to operate in a particular routineand can, at any point in time, be expected to be static when waiting toreceive or to dump a load, loading when actually receiving a load, orroading when driving the vehicle between the roading site and thedumping site.

In the static subroutine of FIG. 3, the software first checks todetermine if all of the subroutines have been executed and whether eachsubroutine has detected a collapsed strut. If all of the routines are inagreement that a strut has collapsed, then control returns to the maincontrol loop without further processing. In this way microprocessor timeis conserved for execution of the main control routine. In decisionblock 86, the value of a variable R is checked. R=0 is an indicationthat not all of the routines have detected a collapsed strut and it isdesirable to execute at least one of the subroutines. Control passes toa decision block 88 which checks to determine if each of the subroutineshave been executed. The variables F11, F22, and F33 are respectively setto the value "1" at the successful completion of the static, loading,and roading subroutines. If all of the subroutines have been executed,control passes to a decision block 90. An unexecuted subroutine resultsin the decision block 90 being bypassed in favor of a decision block 92.In decision block 90, the variables F1, F2, and F3 are compared to thevalue "1". A "1" in these variables is indicative of a collapsed strutbeing respectively detected by the static, loading, and roadingroutines. If the condition F1=F2=F3=1 is satisfied, the variable R isset to 1 and control returns to the main routine via blocks 94,96.Subsequent, execution of the control routine will result in the decisionblock 86 transferring control to a block 98 where the control isimmediately returned to the main routine with none of the subroutinesbeing executed.

Assuming now that not all of the subroutines have detected a collapsedstrut, control will transfer to the decision block 92. In the maincontrol routine, a roading flag will be set in response to either aninput from a speedometer or by strut pressure fluctuations of apreselected magnitude or frequency, either being an indication that thevehicle is actually roading. A value of "1" in the roading flag causescontrol to transfer to the roading routine of FIG. 5. Alternatively, azero in the roading flag is indicative of a static vehicle and controlpasses to block 100 where variables COUNTER, TURN, and F333 are all setto zero. These variables are all used in the roading routine and arereset here in anticipation of the next roading cycle. Controlsubsequently transfers to a decision block 102 in which a variable PASSis checked to determine if the static subroutine is in its first cycleat initial startup. If not, and the variable PASS is equal to the valuezero, control transfers to the loading routine of FIG. 4. On the firstcycle of the static subroutine, control will pass to block 104 where thevariable F11 is set to 1 as an indication that the static subroutine hasbeen performed. Thereafter, in a block 106 the pressure ratios for thediagonally associated vehicle struts 16L,18R;16R,18L are calculated. Forexample, the left front 16L and right rear struts 18R correspondrespectively to the pressures P1 and P4 sampled in the main controlroutine. Similarly, the right front 16R and left rear 18L strutpressures correspond to P2 and P3. The diagonal pressure ratios are usedto enhance the pressure differential caused by a collapsing strut. Acollapsing left front strut will affect the corresponding right rearstrut pressure. Thus, while the pressure differential of the collapsingleft front strut can be detected, the pressure differential of the leftfront right rear ratio will be more significantly affected and easilydetected. Similar reasoning can be applied to each of the struts andtheir corresponding diagonal pressure ratios.

Control passes to a decision block 108 where a determination is madewhether the software is executing a first pass through the staticroutine during the initial startup in the life of the vehicle. Avariable P has initially been set to the value 1 at vehicleinitialization. Thus, on the first pass, control transfers from decisionblock 108 to block 110 where the upper and lower limits of the pressureratio diagonals are set. The upper diagonal limits are set to 125% ofthe previously calculated value while the lower diagonal limits are setto 75% of the previously calculated value. Additionally, the variable Pis set to the value 2. Thereafter, the upper and lower diagonal limitswill remain at the initially calculated value as the variable P has beenset to the value 2 and decision block 108 will bypass block 110.Subsequent to executing block 110, a block 112 returns control to themain control routine.

During the next iteration of the static subroutine, the decision block108 will pass control to a series of decision blocks 114, 116, 118 and120 where the diagonal pressure ratios are compared to theircorresponding upper and lower limits. If the calculated diagonalpressure ratios exceed these limits, control transfers to block 122where the variable PASS is set to the value "2" to prevent the staticroutine from being reexecuted via block 102. Also, variable F1 is set tothe value "1" as an indication that the static routine has detected acollapsed strut. Control passes to a block 124 where control is returnedto the main control routine. If, however, none of the diagonal pressureratios are found to exceed their corresponding limits, then controlpasses to block 126 where the variable PASS is set to the value "2" andsubsequently to a block 128 where control returns to the main controlroutine.

Referring now to FIG. 4 where the loading subroutine is illustrated, andcontrol will ultimately pass after the variable PASS has been set to thevalue "2". In a decision block 129, the variable F222 is checked todetermine if its value is equal to the value "1". The variable F222 isset to the value "1" at the completion of the loading routine and resetby the roading subroutine. If the variable F222 is equal to the value"1", then the subroutine "knows" that the loading subroutine has beenexecuted without a subsequent roading subroutine and there is no valuein reexecuting the loading subroutine. Thus, control transfers to block130 which returns control to the main control routine.

If the variable F222 is equal to the value zero, then control transfersto a decision block 131. In the block 131 a variable named VARIABLE iscompared to the value zero to determine if this cycle is the first of aparticular loading cycle. If the loading software is in the firstiteration of a particular loading cycle, and VARIABLE does equal thevalue zero, then the variable PCLR is set equal to LR and the variablePCRR is set equal to the variable RR in block 132. The variables PCLRand PCRR are indications of the previously calculated left and rightrear strut pressures that correspond to the strut pressure at thebeginning of a particular loading cycle. VARIABLE is reset to the value1 in block 134 to maintain PCLR and PCRR at the values corresponding tothe beginning of the loading cycle and control is returned to the maincontrol routine in block 136.

During subsequent iterations of the loading subroutine, VARIABLE hasbeen reset to the value "1", thus block 131 will pass control to adecision block 138 where the most recent strut pressures LR, RR arecompared to the initial strut pressures PCLR, PCRR. In the illustratedembodiment, a difference of less than 30 psi results in control beingreturned to the main control routine via block 140. It is recognizedthat the values used herein are for illustrative purposes only and areknown to vary between vehicle families. Decision block 138 attempts toensure that a bucket load of material has been added to the vehicle andthat slight changes in the rear strut pressures are not due to minoroscillations of the vehicle suspension. If the presently calculated rearstrut pressures exceeds the initial rear strut pressures by 30 psi, thencontrol transfers to a decision block 142 where the rear pressuredifferential is compared to a 100 psi differential setpoint. Adifferential of greater than 100 psi in either of the rear struts causescontrol to pass to block 144 where a variable INC is incremented by avalue of "1". A decision block 146 receives control from decision block144 and attempts to ensure that the received pressure values are stableby comparing the variable INC to the value 100. If either of the rearstrut pressure differentials have exceeded 100 psi for 100 iterations ofthe loading subroutine, then the control assumes that the recordedpressures are stable and control transfers to block 148. In the block148 variable F22 is set to the value "1" as an indication that theloading subroutine has been successfully completed and the controltransfers to block 150. If the variable INC is less than 100, then thecontrol assumes that the pressure is not stable and returns control tothe main control routine via block 152. If neither differential isgreater than 100 psi, then the control assumes that the data has beentaken at the time a bucket load of material is being dumped into the andlarge oscillations in the pressure different appearing. Thus, controltransfers to block 154 where both INC and VARIABLE are reset to thevalue zero. Control then returns to the main control routine via block156.

In the block 150, the variable LO TEST is calculated as a function ofthe left and right rear strut pressure differential ratio. Moreover, arelationship can be shown to exist between the relative stiffness (k) ofeach strut and the strut pressure differential. This relationship isdefined as follows:

    k=P2**2/(P2-P1)

where P2 corresponds to the current strut pressure; and

P1 corresponds to the previous strut pressure. Because the strutstiffness is dynamic throughout the range of strut movement, it would bedifficult to determine whether a calculated stiffness was within anallowable range. However, each of the rear struts can be expected toreact similarly to additionally loading and, in fact, will be in asimilar range of movement. Therefore, each strut will have a stiffnesssimilar to the other and a ratio of the two will yield a value ofapproximately 1. The equation for such a relationship is as follows:

    kRR/kLR=[(LR-PCLR)*(RR**2)]/[(RR-PCRR)*(LR**2)]

where the ratio of the strut stiffness corresponds to the variable LOTEST. While load distribution has some effect on range of strutmovement, it can be appreciated that there is an upper limit to thiseffect. Any LOTEST value exceeding a 30% differential in strut stiffnesscan be assumed to be attributed to a collapsing strut and therefore inblocks 158 and 160 the LOTEST value is compared to the range 0.7 to 1.3.Any value outside this range transfers control to block 162 where thevariable F2 is set to the value "1" as an indication of a collapsedstrut. Control passes to block 164 and the variable F222 is set to thevalue "1" to prevent the loading subroutine from being reexecuted absenta intervening roading cycle. Further, the variables INC and VARIABLE arealso reset to zero in anticipation of the next loading cycle. Block 166then receives control and transfers that control to the main controlroutine.

Referring now to FIG. 5 where control ultimately transfers in responseto the road flag being set to the value "1" via the decision block 92.In decision block 168 of the roading subroutine, the variable F333 iscompared to the value zero and if not equal to zero causes control topass to block 170 and then return to the main control routine. Thevariable F333 is initially set to the value "1" by the main controlroutine and reset to the value zero during the static subroutine toindicate completion of the static subroutine. Thus, the decision block168 prevents the roading subroutine from being executed prior toexecution of the static subroutine. With the variable F333 set to thevalue zero by the static subroutine, control transfers to a block 171where the variable F222 is set to the value zero to enable the loadingsubroutine to be executed during the next loading cycle.

A variable COUNTER is incremented at block 172 as an indication of theelapsed roading time. Because execution loop time of the subroutines isconsistent, the actual value of the variable counter is an indication ofelapsed time. For example, a counter value equal to 40,000 is equivalentto an elapsed roading time of approximately 6 minutes, 40 seconds.Therefore, in decision block 174, the variable counter is compared to40,000 and if the elapsed time is less than 6 minutes, 40 seconds,control transfers to decision block 176 when the variable TURN iscompared to the value zero. If the variable TURN is equal to the valuezero, then the control assumes that the software routine is on the firstcycle of the roading subroutine and control transfers to block 178 wherethe variable TLF, TRF, TLR and TRR are respectively loaded with thepreviously detected pressures of LF,RF,LR,RR. Additionally, the variableTURN is set to the value "1" and control is returned to the main controlroutine via block 180. As a result of the variable TURN being set to thevalue "1", subsequent iterations of the roading routine result indecision block 176 transferring control to a decision block 182.

In the decision block 182, the left front strut pressure is compared tothe previous left front strut pressure and a differential of greaterthan 30 psi results in a variable CLF being incremented by a value of"1". If the left front pressure differential does not exceed 30 psi,then block 184 is bypassed and the variable CLF is not incremented.Similarly, if the LF pressure is zero, then decision block 183 bypassesthe block 184 and the variable CLF is not incremented. This provisionprevents the variable from being incremented in the event that the strutsuddenly loses pressure and collapses. Allowing the variable to beincremented when the strut is collapsed reduces the count differentialand increases the possibility that the collapsed strut will goundetected. At the end of the six minute, 40 second period, the variableCLF will contain a count of the number of times the differential betweentwo adjacent pressure readings of the left front strut exceeds 30 psi.

Control sequentially passes to blocks 186, 188 and 190 where similaroperations are performed for each of the remaining strut pressures. Thevariables CLF, CRF, CLR and CRR each contain counts corresponding to thenumber of times adjacent pressure readings exceeded a 30 psidifferential in the front struts and a 60 psi differential in the rearstruts. In decision block 192, the previous pressure readings TLF, TRF,TLR and TRR are updated with the most recent pressure readings.Thereafter, block 194 returns control to the main control routine.

This process repeats during the roading routine until such time as thecounter variable value exceeds 40,000. At such time, control transfersto block 196 where the variable F33 is set to the value 1 as anindication of a completed roading subroutine. Control transfers to block198 where the variable CLFRF is set equal to the ratio of the left frontCLF to the right front counts CRF. If the count ratio is within therange of 0.5 to 2, then the roading subroutine assumes that the left andright front struts are not collapsing as they have responded similarlyto similar road conditions. However, should the count ratio exceed thesevalues, then decision blocks 200 and 202 will transfer control to block204 where the variabie F3 is set equal to the value "1" as an indicationof a collapsed strut. Similarly, the count ratio of the left and rightrear struts is stored in the variable CLRRR in block 206. In blocks 208and 210, the rear count ratio is compared to the range 0.5 to 2. If thecount ratio exceeds that preselected range, then control again transfersto the block 204 and the variable F3 is set to the value 1. If the rearcount ratio is within the prescribed limits, then control bypasses theblock 204 and transfers directly to the block 212 where the variableF333 is set to the value 1. F333 prevents the roading subroutine frombeing reexecuted absent an intervening loading cycle. Control istransferred to block 214 and ultimately returns to the main controlroutine.

Industrial Applicability

In the overall operation of the off-highway truck 14, assume that thevehicle 12 is not being operated for the first time in its productivelife, but has previously been used in a typical manner whicn includesthe static, loading, and roading portions of a normal hauling cycle. Atstart-up, when the vehicle is first turned on for daily operation, themain control routine first reads the pressures of each of the struts16L,16R,18L,18R and stores these values in the variables P1,P2,P3,P4. Amain control routine then calls the static subroutine where adetermination is made that none of the subroutines have detected acollapsed strut. On the initial pass through the static subroutine, thediagonal pressure ratios LFRR,RFLR are calculated and compared to thepreviously calculated upper and lower diagonal pressure limits. Ifeither of the diagonal pressure ratios LFRR,RFLR, are outside the upperand lower limits, the variable F1 is set to the value "1" indicating acollapsed strut. The diagonal pressure ratios are computed and comparedto the upper and lower limits only once during initial daily start-up.

While the vehicle operator waits for the vehicle to reach operatingtemperatures and pressures, the main control routine periodicallysamples the pressures of the struts and calls the static subroutine.After the initial cycle of the static subroutine, the loading subroutinewill thereafter be called. At the initial iteration of the loadingroutine, the variables PCLR and PCRR are set to the left and right rearstrut pressures corresponding to an empty truck. Subsequent iterationsof the loading routine compare the most recent strut pressures to theempty truck strut pressures stored in variables PCLR and PCRR. Apressure differential of greater than 100 psi is used to indicate that aload of material has been added to the off-highway truck. To prevent anunstable pressure from being recorded, the loading subroutine will notcalculate the rear strut stiffness ratio until after the vehicle hasstabilized. The loading subroutine requires that the pressuredifferential remain greater than 100 psi for a total of 100 iterations.At this time the response of the left and right rear struts to the addedload are compared to each other and if significantly dissimilar, then acollapsed strut is assumed and the flag F2 is set to the value 1.Independent of whether a collapsed strut is detected, the variable F222is also set to the value "1" to prevent the loading subroutine frombeing reexecuted before an intervening roading subroutine.

However, the instant example, the operator is simply waiting the vehicleto attain operating status and does not at this time expect a load to bedelivered to the of the truck. Therefore, the loading routine beperiodically called; however, the pressure differential will not exceedthe required 100 psi. In the portion of the operating cycle where theoperator drives the vehicle to the loading site, the main routine willrecognize that the vehicle is roading, but an intervening loading cyclehas not occurred and the roading flag will not be set. At the prescribedintervals the main routine will continue to sample the strut pressuresand call the static subroutine. The static subroutine will respond tothe absence of road flag and call the loading subroutine until loadingcycle is performed.

The vehicle will ultimately cease roading as it reaches the loadingsite. Necessarily, as the vehicle is loaded, the rear strut pressurescan be expected to respond with a pressure differential of greater than100 psi as material is loaded onto the truck. When this 100 psi orgreater pressure differential is detected and assumed stable, theloading subroutine will calculate the diagonal strut stiffness andcompare these to the previously discussed limits.

After loading cycle is complete, the operator will road the vehicle tothe dumping site over which time the roading subroutine will once againbe called to make a determination of the status of the struts. Over thatperiod of time, a count is maintained for each of the struts indicatingthe number of times adjacent pressure readings for each strut exceedsthe preselected value. At the end of that period, the counts for thefront struts are compared and used as an indication of a collapsed strutif the counts are significantly different. Similarly, the rear strutcounts are compared to one another and used as an indication of acollapsed rear strut if the counts are significantly different. Forexample, in a vehicle where the struts are properly charged, each of thestruts can be expected to respond to the road and load conditions in amanner similar to that strut on the same axle of the vehicle. Moreover,if one of the struts on an axle is partially collapsed, pressurevariations of significantly smaller magnitude can be expected. Thus, anaxle with a partially collapsed strut will see a significantly lowercount for the collapsed strut and a corresponding greater differentialbetween the struts on that axle.

At the dumping site, after the roading cycle has been completed, thestatic subroutine will periodically transfer control to the loadingsubroutine. The loading subroutine will not be executed until the driverreturns to the loading site and the next loading cycle begins.Thereafter, the process repeats with each roading and loading cycle.

Other aspects, objects and advantages of this invention can be obtainedfrom a study of the drawings, the disclosure, and the appended claims.

We claim:
 1. A method for detecting a collapsed strut of a work vehiclehaving a plurality of left and right strut mounted wheels,comprising:periodically sensing the internal pressure of each of thestruts and delivering a plurality of first signals each having amagnitude correlative to the internal pressure of each respective strut;calculating ratios of the first signals of selected pairs of the struts;comparing the magnitude of each of the ratios to an upper and lowersetpoint and delivering a second signal in response to at least one ofthe ratios being outside the upper and lower setpoints; and delivering athird signal indicative of a collapsed strut in response to receivingthe second signal.
 2. A method, as set forth in claim 1, wherein thestep of calculating the ratios of the first signals of selected pairs ofstruts includes calculating the ratios of the first signals of selectedfront struts relative to the first signals of selected rear struts.
 3. Amethod, as set forth in claim 2, wherein the step of calculating ratiosincludes calculating the ratio of the left front and right rear firstsignals and calculating the ratio of the right front and left rear firstsignals.
 4. A method, as set forth in claim 1, wherein the step ofcomparing includes comparing the magnitude of each of the ratios todistinct corresponding pairs of upper and lower setpoints and deliveringthe second signal in response to the magnitude of at least one of theratios being outside the respective upper and lower setpoints.
 5. Amethod, as set forth in claim 4, including the step of calculating theupper setpoints corresponding to each respective ratio at the beginningof the life of the vehicle by increasing the calculated ratios of thefirst signals by a preselected percentage, and storing the ratios as therespective upper setpoints of each ratio.
 6. A method, as set forth inclaim 4, including the step of calculating the lower setpointscorresponding to each respective ratio at the beginning of the life ofthe vehicle by decreasing the calculated ratios of the first signals bya preselected percentage, and storing the ratios as the respective lowersetpoints of each ratio.
 7. A method, as set forth in claim 1, includingthe step of preventing calculation of the ratios in response to movementof the vehicle.
 8. A method, as set forth in claim 1, including the stepof preventing comparing the ratios in response to movement of thevehicle.